Optimize blueprint.variationMap.equal

blueprint.variationMap.Equal was responsible for 11.5% of allocations
and 2.2% of allocated memory.  reflect.DeepEquals is an expensive way
to compare a map.  variationMap.subsetOf is already iterating through
the elements of the map to compare them, replace equal with a check that
the maps are the same length and then reuse subsetOf to check that the
have the same keys and values.

Test: SOONG_PROFILE_MEM=/tmp/mem.pprof m nothing
Change-Id: Ifb7cdf612e5455fd2f412488b7f94416c4e70c54

context.go

@@ -441,7 +441,10 @@ func (vm variationMap) subsetOf(other variationMap) bool {
 }
 
 func (vm variationMap) equal(other variationMap) bool {
-	return reflect.DeepEqual(vm, other)
+	if len(vm) != len(other) {
+		return false
+	}
+	return vm.subsetOf(other)
 }
 
 type singletonInfo struct {